BP Plc on Tuesday delayed a critical test that will determine if it can close a cap atop its ruptured Macondo well that has leaked oil into the Gulf of Mexico for the last 12 weeks.

The British energy giant and U.S. authorities said they were postponing the test that had been scheduled for Tuesday to establish whether the well can withstand the pressure caused by closing the cap at the wellhead.

"We decided that the process may benefit from additional analysis that will be performed tonight and tomorrow," retired Coast Guard Admiral Thad Allen, who is overseeing the U.S. response to the spill, said in a statement.

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The tests, due to last between six and 48 hours, had been scheduled for late Tuesday on BP's newly installed "capping stack," which has a better seal than the last cap placed on the well and aims to stop oil from spewing out of the failed blowout preventer.

Neither the coast guard nor BP gave any indication about when the tests might now begin.

If the cap, which was put in place on Monday, is not sealed, BP intends to contain the whole of the oil flow by mid-July by siphoning it off through pipes to ships at the surface, Allen had said earlier.

Such developments are keenly eyed by investors as BP's ultimate costs for the massive environmental disaster will likely hinge on how much oil is determined to have flowed freely into the Gulf.

As the oil giant prepared for a potential turning point in the worst offshore spill in U.S. history, it said its plans to sell some non-core assets, which will help pay for a $20 billion clean-up fund, were moving forward.

Both pieces of news had helped BP shares maintain their recent recovery in London, although they seesawed in New York with profit-taking erasing most of the early gains.

The only proven way to permanently kill the leak lies in the drilling of relief wells to intercept the ruptured one. The first of two such wells started in May is expected to intercept it by the end of July and plug it with drilling mud and cement by mid-August.

Photo shows work continuing at the
site of the BP oil well spill in the Gulf of Mexico, in this image
captured from a BP live video feed July 13, 2010.